An end effector is a device connected to the end of a robot arm that interacts with the environment, for example, with an automotive body panel or windshield in an automotive assembly environment, to perform a task. The structure of an end effector and tooling or fixturing included as part of the end effector depends on the task the robot will be performing. The end-effector may include vacuum cup tool modules which are pneumatically actuated. The end effector may have articulating portions, such as an articulating frame, that allow the end-effector to be reconfigurable for different tasks. For example, an end effector with vacuum cup tool modules and an articulating frame can be reconfigured to pick and place automotive body panels and windshields of various configurations, providing flexibility in a manufacturing environment.
A reconfigurable end-effector with an articulating portion, such as an articulating frame, may use a bi-directional rotary locking device, or clutch, including an input shaft rotating an output shaft to rotate the articulating portion of the end-effector frame. However, unlocking the bi-directional clutch with the input shaft and output shaft rotating in the same direction creates the potential for an “overhauling effect” where the output shaft accelerates faster than the input shaft, causing the clutch to chatter or bind. The output shaft can be momentarily accelerated faster than the input shaft by introducing a pre-existing torque or moment load on the output shaft in the same direction as the intended rotation of the output shaft prior to release of the clutch. The pre-existing torque or moment load may be the gravity moment load of the articulating portion of the end effector frame, as the articulating portion rotates from a flat frame position to an articulated position.
A gravity counterbalance can be used to counteract the overhauling effect, and is typically comprised of an extension spring located in the direct tension load path to apply a tension load opposing the gravity moment load of the articulating portion. However, this typical concept of a gravity counterbalance is never perfectly balanced due to variability in the free length of the extension spring. If the spring stiffness is inadequate, the overhauling effect will continue to occur. If the spring is too stiff, the counterbalance will exert unnecessary resistance to the articulation effort.